This invention relates to a visual landing aid for aircraft. In particular it relates to apparatus for use in aligning a projector of a visual landing aid.
A visual landing aid known as the visual approach slope indicator is designed to provide the pilot of an aircraft with a visual indication that he is approaching a runway along the correct path. The slope indicator comprises a number of units each of which is arranged to direct a beam of light along the approach path of an aircraft. The light beam emitted by each unit is a composite one; when it is viewed from above a certain angle of elevation it appears white while when it is viewed from below that elevation angle it appears red. There should be a sharp transition between the red and white at the particular elevation angle. The angle at which the transition occurs is set to that at which an aircraft should be approaching the runway. In one form of slope indicator two such units are located one behind the other on each side of the runway. When the aircraft is approaching the runway at the correct angle the pilot sees the forward unit as white and the rear unit as red. If the aircraft is approaching too low the pilot sees both units as red and immediately has a warning that he is undershooting. If he is too high both units appear white.
In an alternative arrangement a number of units are located in side by side relationship on one or both sides of the runway. Each unit of a group on one side of the runway is arranged such that the elevation angle at which transition from white to red occurs is slightly different from the other units. For example, for an approach angle of 3.degree. the unit closest to the runway can be set at an angle of 3.degree.30', the next unit at an angle of 3.degree.10', the next unit at an angle of 2.degree.50' and the next at an angle of 2.degree.30'. When the aircraft is approaching at the correct angle the pilot will see the two units closest to the runway as red and the outer two units as white.
Originally each unit comprised a housing containing a light source arranged at the focus of a parabolic reflector. Light from the source was reflected by the parabolic reflector through a slit in the front of the housing. A red filter was located in the light beam to provide the composite red and white beam. Subsequently projection units have been designed to provide improved beams. The projection units comprise a light source located at the focus of a parabolic reflector, a lens mounted coaxially with the parabolic reflector and a filter, typically a red passing filter, disposed intermediate the reflector and the lens, the filter being close to the focal point of the lens and terminating on the axis of the reflector and lens. Such a projector will hereinafter be called a projector of the type referred to.
It is important that there is a sharp transition between the red and white in order that the pilot may accurately position the aircraft on the correct approach path. A small variation in the angle of approach can translate into a considerable variation in touch down point along the runway.
It has been found by experiment that to obtain the sharpest transition between red and white portions of the beam, the optimum position of the filter relative to the lens is not, as would be expected, precisely at the focal point of the lens but at a position slightly closer to the lens than the focal point. This position is generally found empirically but this is inconvenient since the projector has to be viewed from a considerable distance. The present invention is concerned with a technique which enables the filter of the projector to be set to the optimum position more conveniently. This involves use of an apparatus which comprises a base having a surface porition for supporting a projector, a lens arranged to focus light from the projector onto an image receiving means and means allowing observation of the image receiving means.